1. Field of Invention
The present invention relates to a method and apparatus for freeze drying or lyophilizing biologicals. More particularly, the present invention relates to freeze drying biologicals of the type that are used as anticoagulants in blood-gas analysis.
2. Description of the Prior Art
The method of first freezing and then drying biologicals by sublimation of ice in a vacuum has been known for many years. Freeze drying, or lyophilization, is similar to ordinary vacuum distillation, with the exception that the material to be dried must be frozen below its eutectic point prior to drying under very low absolute pressure during controlled heat input.
A great deal of freeze drying is done in laboratory research. Commercial applications of the process are widely accepted throughout the health profession because of the greatly enhanced storage capacity of freeze dried biologicals. There are basically three main methods for preparing materials for lyophilization in a freeze dryer.
The first method of preparation involves simply placing the material into a tray and lyophilizing the entire product in bulk. In a second method, the same, or very similar, trays can be utilized to support flasks or serum bottles during the lyophilization process. The third method applies to very small quantities for specialized research. In the third method, ampules are connected to manifolds available on many freeze dryers. The ampules contain the materials without any tray whatsoever.
Trays are therefore well known for use with chamber freeze dryers. Variations of the trays include removable bottoms to enhance heat tranferred during the dry heat portion of the lyophilization process. Wire mesh bottomed trays can be used for solid, undissolved materials having sufficient bulk to span the mesh, or can support bottles of sufficient size. Filter sealed trays are also available that include a top having a filter media incorporated therein for trapping "fly-away" products.
However, none of the trays of the prior art are well suited for producing a final product in the form of individual unit dosages. In fact, the concept of storing a preset dosage of a given biological in a ready-to-use condition has only been recently developed. For example, the reader is referred to U.S. Pat. No. 4,206,768 to Bailey, that patent having common ownership with the present invention. Thus, none of the prior art trays are very well suited for producing a pledget, or single unit dosage, of freeze dried biological material having a preset unit value, units being defined by the United States Pharmacopoeia (U.S.P.).
Heparin is an anticoagulant derived from porcine intestinal mucosa or beef lung. This anticoagulant is extremely valuable in obtaining human blood samples and preventing coagulation of the blood prior to blood-gas analysis or other blood tests. The principal method for utilizing heparin in aspirating a blood sample has been by use of a syringe to draw a set volume of heparin solution from a vial of heparin. This usually involves drawing out more of the heparin than is needed and then expelling the excess. The blood sample is then taken in the normal manner. If the heparin were already in the syringe, ready for use and of a specific concentration, then the procedure would be extremely quick, without the need to measure out any solution.
U.S. Pat. No. 4,257,426, issued 3-24-81, to Bailey, also of common ownership with this application, shows a heparin flake, of preset unit dosage, already placed within the barrel of a syringe. The flake is formed by allowing a solution of heparin to evaporate, and does not therefore give the extended storage life associated with freeze drying.